In an optical record disc of the type presently used with a DAD (Digital Audio Disc player or a video disc player, audio or video information is recorded in optically coded form as a series of recesses or pits formed in the information carrying face of the optical record disc and is arranged in either a single spiral track or a plurality of concentric tracks about the center axis of the optical record disc. The audio or video information thus recorded is reproduced by optically scanning the individual recesses or pits along the spiral track or the concentric tracks by means of a convergent spot of a laser light beam developed from a laser light source. The laser light beam is utilized to detect the lengths and spacings between the scanned recesses or pits. During reproduction, the optical record disc is usually rotated about the center axis thereof at a constant linear velocity (CLV) or a constant angular velocity (CAV), and the convergent spot of the laser light beam is displaced perpendicularly and radially relative to the optical record disc by a focusing device and a tracking device of a pick-up unit. These elements are components of a DAD or video disc player, and are controlled by a focusing servo and a tracking servo, respectively. The laser beam is applied to a selected track location and is either reflected by the information carrying face of the optical record disc or passed through the optical record disc. The beam of light thus reflected by or passed through the optical record disc is then converted into an electrical signal by means of a photoelectric transducer mounted on the pick-up unit, so as to facilitate further conversion into an audio or video signal.
In order to reproduce the information recorded on the optical record disc, the convergent spot of the laser light beam must be precisely focused on the information carrying face of the optical record disc and applied to a selected track location on the information carrying face. To this end, it is known to employ a method in optical record disc information reproducing apparatus wherein light beams, exclusively used for scanning, i.e., for focusing and tracking, are generated separately from the light beam used for reproducing the audio or video information signal. A photoelectric transducer is used for detecting the scanning light beam and effecting scanning control. Another known method involves extracting both the audio or video information signal and the scanning signal with a single light beam.
According to these methods, the light beam for the scanning servo is reflected by the optical record disc and introduced into a photoelectric transducer. The light beam thus reflected is converted into a focusing error information signal and a tracking error information signal. The focusing error information signal is introduced into a focusing actuator for driving an objective lens mounted on the pick-up unit perpendicularly relative to the optical record disc, so as to accurately apply the convergent spot to the information carrying face. The focusing actuator comprises the focusing servo for the convergent spot of the laser light beam.
The focusing servo must have relatively high response characteristics to enable the convergent spot to accurately follow quick fluctuations of the information carrying face which are typical during reproduction conditions. Generally, the gain of the focus servo system is adjusted for adequate response characteristics using the focusing error information signal, which is obtained in a focus search operation performed prior to the disc reproducing operation. That is, at a start of the disc reproducing operation, an objective lens is driven to a position roughly corresponding to the focusing condition by focus search means. Then, the objective lens is kept in a correct focus position by a focus servo means. The focusing error information signal for adjusting the focus servo gain is obtained in the focus search operation at the start of the disc reproducing operation.
FIG. 1 shows an example of conventional focus servo gain setting circuits for an optical record disc reproducing apparatus. In FIG. 1, parts which are not relevant to the control of the apparatus are omitted from the illustration. Optical pickup 11, which is conventional, comprises four photoelectric transducing elements 11a, 11b, 11c and 11d arranged diagonally to each other for separately detecting a reflected light beam, and a circuit for processing signals detected therefrom. The optical pickup 11 generates three signals, an audio or video information carrying signal, i.e., so-called an RF (Radio Frequency; a very high frequency) signal, a tracking information carrying signal, i.e., a tracking error signal, and a focusing information carrying error signal, i.e., a focusing error signal, through the processing circuits. However, for simplicity of operation, only a circuit 12 for generating the focusing error signal FE is shown in FIG. 1. The processing circuit 12 for the focusing error signal FE is constituted by a subtraction circuit.
Signals detected by two photoelectric transducing elements, e.g., 11a and 11c corresponding diagonally to each other are applied to an inversed phase input of the subtraction circuit 12. Other signals detected by two other photoelectric transducing elements, e.g., 11b and 11d corresponding diagonally to each other are applied to a non-inversed phase input of the subtraction circuit 12. Thus, the subtraction circuit 12 produces the focusing error signal FE. The focusing error signal FE is applied through a gain controllable amplifier 18 to a first input terminal 14a of a selector switch 14. The focusing error signal FE is further applied to a level detection circuit 19. The level detection circuit 19 detects the level of the focusing error signal FE and applies its output, i.e., a level detection signal S19 to a control terminal of the gain controllable amplifier 13. Thus, the gain of the gain controllable amplifier 13 is controlled in accordance with the level of the focusing error signal FE. As a result, the focus servo gain of the focus servo system is automatically adjusted.
A second input terminal 14b of the selector switch 14 is coupled to an objective lens drive circuit 15. The objective lens drive circuit 15 for the gain setting operation and the selector switch 14 are coupled to a focus operation control circuit 20. An output terminal 14c of the selector switch 14 is coupled to a focus actuator 18, which is provided for moving an objective lens not shown) in the direction perpendicular to an information carrying face of a disc, through a phase compensation circuit 16 and a drive amplifier 17. In the Figure, the focus actuator 18 is represented by a focus actuator coil.
Referring now to FIGS. 2(a), 2(b) and 2(c), the operation of the conventional focus servo gain setting circuits for an optical record disc reproducing apparatus shown in FIG. 1 will be described. The focus operation control circuit 20 controls the selector switch 14 to select the second input terminal 14b, when a disc reproducing operation is requested. The focus operation control circuit 20 further controls the objective lens drive circuit 15, so that the circuit 15 generates a lens drive signal S15. The lens drive signal S15 alternates at a period Of about 0.5-1.0 sec., so that the signal S15 has a triangular waveform, as shown by waveform S15 in FIG. 2(a). The lens drive signal S15 is applied to the focus actuator coil 18 through the selector switch 14, the phase compensating circuit 16 and the drive amplifier 17. The focus actuator coil 18 actuates the objective lens to move perpendicularly relative to the information carrying face of the optical record disc. As a result, a location Lf of the focal point of the light beam varies in response to the lens drive signal S15. A graph Lf in FIG. 2(b) shows the variation of the location Lf of the focal point. In FIG. 2(b), a line F shows the location of the information carrying face. When the location Lf of the focal point moves across the location F of the information carrying face, the optical pickup 11 generates the focusing error signal FE, as shown by a graph FE in FIG. 2(c).
The level of the focusing error signal FE is detected by the level detection circuit 19 each time the focal point moves across the location F of the information carrying face. The output of the level detection circuit 19, i.e., the level detection signal S19 responsive to the focusing error signal FE is applied to the gain controllable amplifier 13, so that the gain of the gain controllable amplifier 13 is set to an adequate level. The gain setting of the gain controllable amplifier 13 is completed through several applications, e.g., six times or more, of the level detection signal S19. The focus operation control circuit 20 or any other conventional circuit, e.g., a microcomputer, counts the applications of the level detection signal S19.
After completion of the gain setting, a focus search operation is performed in a conventional manner. After the focus search operation, the focus operation control circuit 20 controls the selector switch 14 to select the first input terminal 14a. As a result, the focusing error signal FE is applied to the focus actuator coil 18, in place of the lens drive signal S15 from the objective lens drive circuit 15. After that, the objective lens is controlled by the focusing error signal FE, so that the focus servo for the objective lens is controlled according to the focus servo gain previously set.
However, the conventional focus servo gain setting circuit for an optical record disc reproducing apparatus, as shown in FIG. 1, has a drawback in that it takes a long time for the gain setting operation.
In particular, as described above, the gain setting of the gain controllable amplifier 13 is completed through several applications, e.g., six times or more of the level detection signals S19. The focusing error signal FE is generated only twice in every cycle of the oscillatory movement of the objective lens during the gain setting operation. On the other hand, the oscillatory movement of the objective lens has a relatively long cycle, e.g., 0.5-1.0 sec., as mentioned before. Thus, the gain setting of the gain controllable amplifier 13 takes a considerably long time, e.g., about 1.5 sec. or more.
It should be apparent to those knowledgeable in the field of scanning beam control systems, and particularly the field of scanning beam tracking systems, that there is a need for a system that can detect and compensate for deviations from normal operation of the specified characteristic being controlled. In particular, it should be apparent from the foregoing that a need exists for a focusing system that includes means for detecting and compensating for record medium defects that otherwise can cause a loss of focusing. The present invention fulfills this need.